The present invention is directed to a mobile radio network having a plurality of base stations in a spatial arrangement in the fashion of a cellular system in a hierarchic cell structure, composed of a macro cell and of at least one micro cell.
The hand over from the radio management domain of one cell into that of a neighboring cell can be initiated for various reasons. For example, hand over reasons can be that the received level has fallen below a certain threshold, the quality (bit error rate--BER) is too poor, the distance to the administering base station has become too great, because of excessively high interference, because of the power budget of a better cell or for traffic reasons as well. These are thereby a matter of what are referred to as static hand over decisions, i.e. the data measured by the mobile station and/or base station, namely level, quality, distance, etc., or the quantities derived from the measured data are compared, upon utilization of a filter, for example by averaging the measured data, to chronologically constant thresholds that can be administered by O & M (operation and maintenance). This procedure is shown in a block circuit diagram of FIG. 1. The measured data arriving from a mobile station and/or base station are supplied to a filter which is followed by a means for the hand over decision process which outputs a hand over request given the presence of appropriate criteria.
In order to have both channel capacities in regions having high subscriber densities as well as an adequate supply available between regions of high subscriber density, a switch shall be made to mixed or hierarchic cell structures composed of small cells (micro cells) which are in turn integrated in a large cell (macro cell, also referred to as umbrella cell). FIG. 2 shows such a structure. A plurality of micro cells MC1, MC2, MC3 having the base stations BS1, BS2, BS3 are thereby arranged within a macro cell having the base station BS. There are four types of hand over for a mobile station that moves in such a mixed cell system, namely from the macro cell to another macro cell, from the macro cell to a micro cell, from one micro cell to another micro cell as well as from the micro cell to a macro cell. The radius of the micro cells will thereby be extremely small and usually amount to only a few hundred meters. Slowly moving mobile radio telephone users are to be supplied by the micro cells; the macro cells (umbrella cells), by contrast, are intended to assure the radio management of rapidly moving mobile stations.
By joining micro cells which are integrated in the macro cell to one another, there is the possibility that a rapidly moving mobile station that is allocated to the base station of the macro cell and travels into the radio management or coverage domain of a micro cell located in the macro cell is allocated to the base station of the micro cell on the basis of static hand over decisions. Since the mobile station moves very rapidly and the micro cell has a small diameter, the mobile station will in turn rapidly leave the micro cell and a new hand over is pending. Either a hand over for the mobile station into the original macro cell or in hand over into the adjoining micro cell will thereby be requested when a further micro cell adjoins the micro cell that has been traversed. FIG. 3 shows such a scenario of a mobile station in a hierarchic cell structure that can be arbitrarily continued. A series of micro cells having the base station BS.sub.Ma, BS.sub.Mb, BS.sub.Mc, BS.sub.Md, BS.sub.Me and BS.sub.Mf are thereby integrated within a macro cell having the base station BS.sub.Macro. A mobile station MS moves along a travel path from point a to point g via the points b, c, d, e and f that lie in the edge region of a few micro cells. The mobile station is thereby first allocated to the macro cell having the base station BS.sub.Macro and sets up a call. It then travels into the radio coverage domain of the micro cell having the base station BS.sub.Ma. The hand over decision process requests a hand over for the mobile station into the micro cell having the base station BS.sub.Ma. The mobile station MS then travels into the radio coverage domain of the micro cell having the base station BS.sub.Mb. The hand over decision process requests a hand over for the mobile station into the micro cell having the base station BS.sub.Mb. After this, the mobile station leaves the radio coverage domain of the micro cell having the base station BS.sub.Mb. The hand over decision process now requests a hand over for the mobile station into the macro cell having the base station BS.sub.Macro. The further micro cells lie outside the travel path of the mobile station, so that the hand over decision process only requests a hand over for the mobile station into the micro cell having the base station BS.sub.Mf when the mobile station travels into the radio coverage domain of the micro cell having the base station BS.sub.Mf. When the radio coverage domain of the micro cell having the base station BS.sub.Mf is left, the hand over decision process requests a hand over for the mobile station into the macro cell having the base station BS.sub.Macro. The mobile station ends the call and remains allocated to the macro cell having the base station BS.sub.Macro. All this requires a high signaling outlay. It can also be stated that the continuous hand over from one micro cell to another can lead to a loss of the call given a slow execution of the hand over decisions, for example given a great averaging length of the measured data.
The speed of the mobile radio telephone subscriber is not taken into consideration in prior methods for deciding a hand over for a mobile station. The hand over methods are based on static hand over decisions, that is, the data measured by the mobile station and/or base station or quantities derived from measured data are supplied to the hand over decision process upon utilization of a filter. In this decision process, the filtered data are compared to permanently set thresholds or are compared to chronologically constant thresholds that can be administered by O & M. When the filtered data downwardly or upwardly transgress the appropriate thresholds, then a hand over is requested for the appertaining mobile station.